Question about temp control

I've got a okie joe longhorn which have a few mods, I've raised the grate in the fire box 3" and extended the exhaust inside the smoker and added a 90 bend and brought it down to grate level. I've also added 2 thermometers to the lid which are at grate level (cheap thermometers but passed the boiling water test) so my question is I smoked an 11lb pork shoulder Saturday and I could not get the grate level temp above 250 which is ok IMO because I like to keep it around 225 250 anyway but I had the air intake wide open and the exhaust wide open. It still would not heat up passed 250 @ grate level. Any ideas ?

There is a thermometer in the top of the lid that came with the smoker and it reads a little high (going off of water boil test) and it reads about 300 when the two at grate level read 225 250, my theory is the hole that the heat passes through is so big the heat hits the top of the smoker and makes its way out the exhaust, what do you guys think ?

No I don't have any diverter plates or a tuning plate installed, I thought about making one that runs the length of the cooking chamber with smaller holes close to the fire box and larger holes on the exhaust side I just don't want to kill the temp all together and ill be sealing it all up maybe next week

The convection plate that you mentioned with the holes that get progressively larger is a good way to go. Be sure that the plate only extends about 2/3 of the way across the CC. I would also suggest that you start your fire with the lump and establish a good coal bed and then switch to splits. Be sure to always keep a good coal bed. Don't let your splits burn down before putting more on. Also, if you can, pre-heat the splits. Put them on top of the FB or somewhere that they can get warm. That way they will ignite quicker.